Conventional optical fibers are fabricated from quartz glass and plastics. Optical fibers produced from quartz glass have a high ability to transmit light without loss and are currently used for long-distance communications. Plastic optical fibers are not as high as quartz optical fibers in their ability to transmit light, but because of their high flexibility, light weight and good processability, their application in short-distance communication light guides or sensors is being studied.
Some applications of plastic optical fibers require high heat-resistance. For example, optical fibers used in an automobile optical data link system must withstand the heat from an engine compartment that often has a temperature as high as 100.degree. to 120.degree. C. However, most conventional plastic optical fibers comprises a core of polystyrene as disclosed in Japanese Patent Publication No. 47695/77 or polymethyl methacrylate as disclosed in British Pat. No. 1,037,498 and U.S. Pat. Nos. 3,993,834, 3,930,103 and 4,161,500 and, therefore, their maximum use temperature limit is as low as about 80.degree. C. At temperatures higher than above 80.degree. C., these optical fibers shrink and their ability to transmit light decreases, and at even higher temperatures, i.e., 100.degree. C. or more, these optical fibers further shrink and may break to make light transmission impossible.
In order to minimize such thermal shrinkage during use, plastic fibers are sometimes previously subjected to heat treatment. This heat treatment is effective in reducing the heat shrinkage of the fibers, but the fibers themselves are no longer flexible and easily break due to vibration or bending.
The present inventors have conducted research to eliminate these defects of conventional plastic optical fibers and noted that the glass transition point and melting point are important physical properties that indicate the performance of polymethyl methacrylate (used as the core) and a fluorine-containing polymer (cladding) at high temperatures. Polymethyl methacrylate used as a core material has a glass transition point of about 105.degree. C. Studies on the relationship between temperature and heat shrinkage of a fiber comprising a core of polymethyl methacrylate show that at temperatures lower than 105.degree. C. the fiber shrinks only slightly, whereas at temperatures higher than 105.degree. C. the fiber shrinks in a very short period of time. For example, at 120.degree. C. the shrink ratio is more than 30%. The major cause of shrinkage of the fiber is the heat relaxation of the polymethyl methacrylate molecules that have been oriented by a stretching effected during or after spinning to provide the plastic optical fiber with flexibility and bending properties. Stretching is essential to provide flexibility and bending properties to fibers of non-crystalline polymers, such as polymethyl methacrylate and polystyrene, and fibers having practicably usable mechanical characteristics cannot be produced without this step. As earlier described, it is possible to heat-set virgin fiber to relax the orientation of the fiber and eliminate the possibility of thermal shrinkage during use, but this impairs the flexibility and bending properties of the fiber.
An alternative method to produce a fiber that will not shrink at temperatures higher than 100.degree. C. is to use a core made of a polymer having a high glass transition point. There are many monomers that are known to provide polymers having high glass transition points and they include styrene derivatives and methacrylate ester derivatives. However, fibers using a core of addition polymers prepared from these monomers have low flexibility and bending properties, and none of them is suitable for practical use.
Condensation polymers are also known to provide high glass transition points, and polycarbonates are condensation polymers that have high transparency and a glass transition point of about 145.degree. C., as disclosed in Japanese Patent Publication No. 43388/76.
Fibers fabricated using a polycarbonate as a core have good thermal characteristics but their ability to transmit light without loss is much lower than fibers of polystyrene or polymethyl methacrylate. This is probably because a polycarbonate produced by condensation polymerization must be free of NaCl and other by-products but it is unavoidable that such impurities remain in or enter the product.
Other non-crystalline condensation polymers which have high glass transition points are polysulfones and polyacryl esters, but high temperatures are necessary to process them into fibers, but polymer decomposition or the presence of impurities is unavoidable and transparent fibers cannot be prepared.
The cladding of a plastic optical fiber is produced from a material having a lower refractive index than the core material. The cladding of conventional plastic optical fibers comprising a polymethacrylate core is produced from a fluoroalkyl methacrylate polymer as disclosed in Japanese Patent Publication No. 8978/68 or a copolymer of vinylidene fluoride and tetrafluoroethylene with a vinylidene fluorine content of, e.g., 77 mol%, as disclosed in Japanese Patent Publication No. 21660/78. However, fluoroalkyl methacrylate polymers start to decompose at low temperature, which is not desired in fiber making, and their glass transition point is in the low temperature range of from 60.degree. to 90.degree. C. Therefore, if a fiber comprising a cladding of a fluoroalkyl methacrylate polymer is subjected to a temperature close to its glass transition point, the transmission loss gradually increases and at temperatures higher than 100.degree. C., the loss is so great that the fiber is no longer usable. A copolymer of vinylidene fluoride and tetrafluoroethylene with a vinylidene fluoride content of 77 mol% has a melting point of 110.degree. C., and, therefore, if a fiber comprising a cladding of such copolymer is subjected to a temperature close to or beyond its melting point, the transmission loss is increased, and, eventually, the cladding will be fluidized to result in a great loss in the transmission characteristics.